In the industrial world a continuous need exists for photosensitive image-forming materials with increased sensitivity and image-quality. These characteristics are related which means that influencing one parameter has its consequences for the other parameter and vice versa. This relationship can clearly be experienced in silver halide photographic materials. An increase of sensitivity of a photographic silver halide material can for instance be realised by an increase of the mean size of the silver halide grains contained in the emulsion of the said material which in turn often results in a decrease of sharpness or contrast of the said material. This particular problem can for instance be solved by increasing the efficiency for electron trapping of the latent-image of a silver halide grain which can be realised by chemical sensitization in the presence of a compound which increases the electron-trapping efficiency mentioned hereinbefore. This compound can be for instance a sulphur compound, a gold salt or a combination of both. It has been noticed in recent years that chemical sensitization with a chalcogenic compound other than sulphur is increasingly practicized in order to enhance photographic sensitivity. These compounds in turn often give rise to an increasing tendency of fog formation and often to stability problems.
Looking at these problems which are for instance related to selenium sensitization various suggestions have already been formulated in the patent literature. Examples of selenium compounds which were suggested for use in order to get an increased sensitivity were substituted selenourea (as described e.g. in EP-A 0 280 031, EP-A 0 458 278), selenoethers (e.g. in JP-A 2-132434, JP-A 4-271341, JP-A 6-175258, U.S. Pat. No. 5,532,120), diselenides (e.g. in EP-A 0 703 492), selenoesters (e.g. in U.S. Pat. No. 5,306,613), selenoketones (e.g. in EP-A 0 476 345), phosphineselenides (e.g. in EP-A 0 506 009) and various others. However as mentioned hereinbefore selenium sensitization easily causes fog as a result of the depth of the electron trap of the formed sensitivity speck which is larger than the depth of the sensitivity speck created by sulphur sensitization. At the same time the stability and the gradation of a selenium sensitized emulsion decrease if no precautions are taken. An alternative way is described by G. J. Burgmaier et al in U.S. Pat. No. 4,810,626 which provides a silver halide photographic material comprising a sensitizing amount of a tetrasubstituted urea compound with the following general structure: N(A.sub.1 R.sub.1) (A.sub.2 R.sub.2)-C(=X)-N(A.sub.3 R.sub.3) (A.sub.4 R.sub.4) where X equals S, Se or Te and where S is preferred (due to the greater solubility of the thiourea compound in aqueous solutions where silver halide sensitization occurs and also to the ready availability of starting materials for thiourea synthesis). In this formula at least one of A.sub.1 R.sub.1 to A.sub.4 R.sub.4 contains a nucleophilic group bonded to the urea nitrogen atom through a 2 or 3 member chain while each of R.sub.1, R.sub.2, R.sub.3 and R.sub.4 independently represents an alkylene, cycloalkylene, carbocyclic arylene, heterocyclic arylene, alkarylene or aralkylene group and each of A.sub.1, A.sub.2, A.sub.3 and A.sub.4 independently is hydrogen or represents a carbocyclic, sulfinic, sulfonic, hydroxamic, mercapto, sulfonamido or a primary or secondary amino nucleophilic group. The chalcogenic atom in the urea compounds of the Burgmaier patent is activated by the nucleophilic group which is generally called a neighbouring group. This type of reactions is summarized by the term `neighbouring group participation`, describing all the intramolecular reactions and all the reactions which involve non-electrostatic through-space interactions between groups within the same molecule. Another example is in principle also described in EP-A 0 458 278. Further DE 196.35.096 and DE 196.48.008 describe the use of chemical selenium sensitization combined with a spectral sensitization with a well-defined sensitizer structure. The group of chemical selenium sensitizers used in these applications shows different chemical structures as already mentioned hereinbefore and does not show the sensitivity-fog relationship which is needed for the new photographic materials of tomorrow. In U.S. Pat. No. 4,810,626, EP-A 0 458 278 and in both DE-applications as well, substantial increase of sensitivity can be realized with the chemical sensitizers mentioned but like in the other patent proposals described hereinbefore no clear demonstration is given that fog could be kept under control. Accordingly there still exists a strong demand for chemical chalcogenic sensitizers which give a deeper electron trap than the corresponding S-sensitizers but in combination with an acceptable low fog level.